Definition of Electron Affinity

In simple words, the likelihood of a neutral atom to gain an electron is known as electron affinity. So, the definition for electron affinity can be, the change in energy, which is measured in kJ/mole, due to the addition of an electron to any neutral atom, which is in the gaseous state, in order to form a negative ion. In context to chemistry and atomic physics, the term electron affinity denoted as E_ea, for an atom or can be for a molecule, is the amount of energy that is released or spent, due to the addition of one neutral atom or molecule, which is in their gaseous state, in order to release a negative ion and some amount of energy. It can be represented as:

X + e⁻ → X⁻ + energy

The equation that shows the electron affinity is equal to the negative change in the energy is:

E_EA = -ΔE

Where, E_EA is the electron affinity and ΔE is the change in energy.

In case of solid state physics, the definition of election affinity is a little different than what is mentioned in case of chemistry. Typically, electron affinity for a semiconductor vacuum interface, which is the semiconductor surface, is denoted by X or E_EA. The definition in case of solid state is, the energy that is obtained by moving an electron, which is present in the vacuum or outside the semiconductor, to the base of the conduction band, which is situated inside the semiconductor. The representation of electron affinity in case of solid state physics is shown below:

The property of electron affinity is only used to measure the atoms and molecule when they are in their gaseous state because, in case of solid state and liquid state the energy level can change if there is any interaction with other atoms or molecules.  Robert S. Mulliken first used a list of electron affinities in order to develop an electronegativity scale that can be used for atoms.

The usual expression for calculating E_ea when an electron is attached is:

E_ea = (E_initial − E_final)_attach = −ΔE (attach)

The expression follows the conversion:

ΔX = X (final) – X (initial) since − ΔE = − (E (final) – E (initial)) = E (initial) – E (final)

The reaction is exothermic in nature because, there is some energy released in the process of gaining one electron and forming the negative ion. This suggests that, the change in energy is negative and thus, the electron affinity is positive. For an example, the electron affinity of Chlorine has a negative sign, which suggests the energy that is being released to add one electron to an atom. The giving up of the energy is represented using a negative sign.

Cl (g) + e^- à Cl^- (g) ΔE = -349 kJ / mol

E_EA = -ΔE

E_EA = +349 kJ / mol

Based on the observation, the conclusion can be made that higher the electron affinity the higher is the chances of an atom to accept electrons and a lower electron affinity means that the chances of acceptance of electron for an atom are very low.